Heartbeat renew failures were only logged, so under DB flakiness a job could
be leased twice concurrently — the original worker kept crawling past a lapsed
lease while another re-leased it. Track consecutive renew failures and, after
MAX_HEARTBEAT_RENEW_FAILURES, signal the worker (via a CancellationToken) to
abandon the in-flight dispatch instead of working a job it no longer owns.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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